Chicken with a large swellen mass on its chest

One of my 3.5 month old chicks has a large, golf ball+ sized mass on one side of her chest. It's not solid, but feels filled with liquid. She seems alright otherwise. My first thought is an infection. If it is, it's not to late in the day for me to get some antibiotics from my vet. Any ideas would be greatly appreciated.

I'm also ashamed because I've had chickens for nearly 30 years, though I have new chicks now because all my other girls were sadly slaughtered by a mink that got into their house last winter. That was bad.